1. Field of the Invention
The present invention relates to a structure of a circuit board that improves transmission quality of high frequency signals, and in particular to an attenuation reduction structure for high frequency signal contact pads of the circuit board, which includes an expanded thickness between a contact pad mounting zone and a grounding layer of a substrate to reduce a capacitance effect between the high frequency signal contact pads and the grounding layer so as to reduce attenuation occurring in the transmission of high frequency signals.
2. The Related Arts
In all sorts of electronic device that are currently available and used, the amount of data transmitting through signal lines is increasingly expanding and consequently, the number of signal transmission lines needed is constantly increased and the frequencies used to transmit signals are also constantly raised.
A number of ways are used to suppress the generation of noises in high frequency signal lines. A common mode chock is commonly used in the known circuits to suppress common mode noises, but it is not fit for applications of circuit boards of high speed/high frequency signals.
In the high frequency signal transmission technology, generally, two high frequency signal lines are grouped as a signal pair to respectively transmit signals of identical amplitudes but opposite phases. Common mode noises that are induced by external interference signals in the two signal lines are of identical amplitudes and identical phases and would be rejected by a differential input pair of an integrated circuit so that the circuit shows a better effect of suppressing electromagnetic interference.
Although the known high frequency signal transmission technology is effective in remarkably alleviating potential problems occurring in the transmission of signals, yet poor designs may result in potential problems in actual applications, such as signal reflection, radiation of electromagnetic signals, loss of transmitted signals, and distortion of waveforms of signals. Specifically, for flexible circuit boards having a substrate that includes a reduced thickness, these problems associated with signal transmission get even worse. Causes for such problems include for example poor match of characteristic impedance in a length extension direction of high frequency signal lines, poor control of additional parasitic capacitance between high frequency signal lines and a grounding layer, poor control of additional parasitic capacitance between a contact pad mounting zone and a grounding layer, and mismatch of characteristic impedance of high frequency signal lines and a contact pad mounting zone.
Currently, various solutions have been proposed to overcome the problems of electromagnetic interference and impedance match in the length extension direction of high frequency signal lines of a flexible circuit board. However, up to date, due to constraints imposed by line widths of high frequency signal lines (the line widths being extremely small) and dimension specifications of signal terminal pins and components of a connector (which are relatively large as compared to the line widths of the signal lines), no manufacturer in this technical field has proposed an effective solution to ensure transmission quality of high frequency signals for a connection between high frequency signal lines and a contact pad mounting zone of a flexible circuit board and an adjacent site thereof.